i would like to set up that another site should open up within my window. meaning my navigation bar and header should stay there but the other site should open inside.

also i wanted to know if i can set up a link that it will automatically fill in the username and password and sign itself in?

For the first question, you would have to do a PHP include. If you know anything about PHP, you'll know how to do it. If not, go read a tut.
For the second question, I'm not sure what you want to do. Please explain this in mroe detail or give me an example.

Also, in the future, please try to search on google before asking us questions which are answered many times elsewhere.